1. Field of the Invention
The invention relates to an electromagnetic deflection unit for a cathode-ray tube, comprising: a hollow annular support provided with a narrow end and a wide end and with a longitudinal axis;
a respective flange at the narrow end and at the wide end of the support, these flanges each having at least one tangential groove with a bottom portion and each having a plurality of mainly radial grooves which merge into a said tangential groove and have at least in the flange at the narrow end a longitudinal extending portion having a width and a depth, which longitudinally extending portions are tangent to an inscribed circle; PA0 a first set of deflection coils for line deflection of an electron beam in a first direction transverse to the longitudinal axis, these deflection coils being wound directly onto the support on the inner side thereof and the turns thereof each traversing the tangential groove and radial grooves of the flanges; and PA0 a second set of deflection coils for field deflection of an electron beam in a direction transverse to the longitudinal axis and transverse to the first direction, these deflection coils being wound directly onto the support and the turns thereof traversing radial grooves in the flanges. PA0 the width and the depth of the longitudinally extending portion of each of the radial grooves are chosen so that the turns traversing these grooves substantially fill these portions, and PA0 the bottom portion of the said at least one tangential groove is located on the surface of an oval cylinder.
Such a deflection unit is known from EP 0 No. 102 658 A1
2. Description of the Related Art
Cathode-ray tubes have a neck-shaped part, in one end of which an electron gun is arranged and the other end of which passes into a conical part, which is joined by a screen. An electromagnetic deflection unit is arranged around the neck-shaped part and against the conical part or at a short distance therefrom. This deflection unit has to be able to deflect electron beams also to the corners of the screen. Therefore, these beams must be prevented from touching on their way to the screen the inner wall of the tube, as otherwise they would be reflected and would reach the screen elsewhere. These undesired reflections can be avoided if the deflection unit is situated sufficiently close to the screen of the tube. However variations in the outer dimension of the conical part of the tube and in the inner dimension of the deflection unit can result in that the deflection unit can be displaced over a smaller distance towards the screen than was expected. Also in this case, it must be possible that the electron beams are correctly deflected to the corners of the screen.
Therefore, it is of great importance that the design disign of a deflection unit provide a large distance between a foremost position of the deflection unit, in which this unit abuts against the conical part of the tube and a correct deflection takes place, and a hindmost position, more remote from the screen of the tube, in which a reflection of electron beam on their way to corners of the screen must not take place. In other words: the deflection unit must have a large axial sliding tolerance. It has been found that the known deflection unit has a comparatively small axial sliding tolerance. A large axial sliding tolerance is also of importance in order to neutralize, for example, variations in the properties of the electron gun.
The design of a deflection unit can also lead to errors of different nature, such as raster errors, astigmatic errors and coma errors. These errors can be corrected by different means. Coma errors can be corrected most effectively by adaptation of those parts of the deflection unit which are located at the narrow end of the support close to the electron gun. Areas more remote from the electron gun can then be employed for correction of other errors. The possibilities for correction of errors are therefore greater when the deflection unit extends in closer proximity of the electron gun. However, this has the disadvantage that the deflection unit begins to deflect electron beams already in the proximity of the electron gun, as a result of which the risk of collisions with the wall of the tube and undesired reflections increases. The axial sliding tolerance of the deflection unit is therefore smaller in a unit which extends in closer proximity of the electron gun.